Clamps of this type generally embody opposed levers of the first class with a common fulcrum near the railhead gripping jaws of the levers in pincer-like fashion but, unlike common pincers, the levers do not cross each other at the pivot. A hydraulic cylinder and piston is arranged to pull the longer arms of the opposed levers toward each other and thereby move the jaws apart to release the clamp from its grip on the railhead. This happens when the motors which drive the gantry are energized to move the gantry. The longer arms of the levers are connected by a toggle link, the common pivot for which is forced down when the levers are pulled together by the hydraulic unit. This downward travel of the toggle pin is opposed by springs so that when pressure falls in the hydraulic system, as when the gantry is stopped at a selected location, the springs act against the toggle pin and links in the opposite direction to move the levers apart and "set" the jaws against the railhead.
Under the conditions of use where stopping and starting occurs many times in a day and with the massive equipment involved, the clamping surfaces of these clamping jaws are provided with railhead-contacting inserts to protect the jaws themselves from the rapid wear to which they would otherwise be subjected. These inserts wear relatively rapidly but they are removably held in the jaws by bolts and they are replaceable, but adjustment of the jaws for wear of the inserts can only be done when the clamp is set and the gantry secure against motion and is difficult to accomplish. There is no way to readily indicate the amount of wear of the inserts that has taken place or take steps to make adjustment, and there is no ready way to determine when wear has become so extensive as to require replacement of the inserts.
It may also happen that, for some reason, the hydraulic pump may fail to release the clamp and the massive and expensive equipment must stand idle, and urgent and costly work may be delayed while workmen try to release the clamp or repair the hydraulic system or, perhaps, the circuits that control its operation.
A primary object of the present invention is to provide an automatic adjustment for wear in the clamp inserts and, at the same time, disclose on an easily visible scale the total extent of wear that has taken place and thus allow for the use of the gantry to be scheduled for the replacement of the inserts when needed.
A second important object of the present invention is to provide manually operable means for operating the clamping jaws if the hydraulic system does not function so that the gantry may be moved from one position and clamped without having to immobilize the gantry for long periods of time while the trouble with the hydraulic system is located and repaired.
Briefly, these objects are achieved with only relatively inexpensive changes in clamps of this type now in use, of which one manufactured by my assignee, Dravo Corporation, of Pittsburgh, Pa., is an example.